1. Field of the Invention
The present invention relates to a superconductor device using a superconductor of compound oxide, and more specifically to an electrode for electric connection to a compound oxide superconductor which can be effectively implemented in a superconductor device using the compound oxide superconductor, as well as a method for forming the same electrode.
2. Description of Related Art
In applications of various types of compound oxide superconductors (called simply "oxide superconductor" hereinafter), superconductor electronic devices and superconductor wirings for electronic devices are ones of fields most hopefully expected to be put into practical use. Josephson devices, SQUIDs, superconductor transistors and superconductor circuit wirings formed of oxide superconductors have been already reported.
In general, superconductor devices include superconductive conductors or wirings which allow a so-called superconducting current to flow therethrough. However, in all of superconductor circuits and devices, the superconductive conductors or wirings have to be electrically connected to circuits or devices which operate under a normal conduction condition.
For this purpose, in the above mentioned superconductor devices, an electrical connection has been realized by using a thin metal wire such as an Au which is called a "bonding wire". If the superconductor is a metal superconductive material, the bonding wire can be fixed and electrically connected directly to a portion of the metal superconductor. However, if the superconductor is an oxide superconductive material, it is difficult to fix or secure the bonding wire to a portion of the oxide superconductor. In the case of the oxide superconductor, therefore, a metal electrode has been deposited on a portion of the superconductor by means of vacuum evaporation of noble metal such as Au (gold), and thereafter, a bonding wire has been fixed and electrically connected to the metal electrode thus formed on the oxide superconductor. Since the noble metal typified by Au is very low in reactivity, it will not give an adverse effect to the oxide superconductor. In addition, even if the noble metal typified by Au is in contact with air, it is hardly oxidized. In this point, the noble metal typified by Au is suitable for the electrode for the oxide superconductor.
However, the noble metal typified by Au does not have a good adhesion or bonding property to the oxide superconductor, and therefore, a contact resistance has often become large. Therefore, the superconductor device in which only a very small amount of electric current is flowed has become unstable in operation, and cannot often exert an expected performance.
Furthermore, when a metal electrode is formed on a portion of an oxide superconductor, after a metal film is deposited on an oxide superconductor thin film, the metal film is patterned. It has been an ordinary practice to perform the patterning by using a photolithography.
The following is one example of a "lift-off" process for forming a metal electrode on a thin film of oxide superconductor.
First, a thin film of oxide superconductor is formed on a substrate, which has been properly selected dependently upon the kind of an oxide superconductor to be formed. For example, substrate is formed of MgO. In addition, the film of oxide superconductor is deposited by means of sputtering, MBE (molecular beam epitaxy), CVD (chemical vapor deposition) or other suitable process.
Then, a photoresist layer is deposited on the thin film of oxide superconductor, and patterned so that an opening for allowing deposition of metal electrode is formed in the deposited photoresist layer. In the opening of the patterned photoresist layer, the thin film of oxide superconductor is exposed.
Furthermore, metal is deposited by, for example, vacuum evaporation, so that the metal is deposited directly on the thin film of oxide superconductor exposed in the opening of the pattern photoresist layer.
Thereafter, the photoresist layer is removed, so that the metal layer deposited on the photoresist layer is removed together. Thus, the metal layer remains only on a position of the thin film of oxide superconductor corresponding to the opening of the photoresist layer. Namely, a metal electrode having a configuration corresponding to the opening of the photoresist layer is formed on the thin film of oxide superconductor.
However, the above mentioned conventional metal electrode forming method is disadvantageous in that, since the photoresist layer is deposited directly on a surface of the thin film of oxide superconductor, an interfacial reaction occurs, and therefore, the characteristics of the oxide superconductor is deteriorated. In addition, in the process of the photolithography, since the oxide superconductor is in contact with an alkaline developing liquid and a cleaning water, the characteristics of the oxide superconductor is further deteriorated.